The resolution could be vetoed by Russia, which backs the Assad regime and has claimed the deaths were caused by a “rebel stockpile” of sarin that was blown up in an explosion.

Theresa May in Saudi ArabiaCredit:
Simon Dawson/Bloomberg

Mrs May has also called for an investigation by the Organisation for the Prohibition of Chemical Weapons.

Asked whether Britain might respond by bombing targets in Syria, a Downing Street official said: “No one is talking about military action.”

Boris Johnson, the Foreign Secretary, on Wednesday morning repeated Mrs May’s stance that Mr Assad should not be allowed to remain in power after the Syrian conflict is over, and called for criminal prosecutions of those behind the gas attack.

Boris: Assad must pay price for chemical weapons attack

00:59

Arriving at a meeting of EU foreign ministers in Brussels, Mr Johnson said: “This is a barbaric regime that has made it impossible for us to imagine them continuing to be an authority over the people of Syria after this conflict is over.

“It will need to be investigated and the question of culpability for what has been done will need to be established and where it is possible to bring criminal prosecutions I think that is certainly appropriate.”

Arriving for a major aid-pledging conference for Syria in Brussels, Mr Johnson told reporters: "I've seen absolutely nothing to suggest, or rather to lead us to think, that it's anything but the regime.

"All the evidence I have - and there may be more to come out of this - all the evidence I've seen suggests that this was the Assad regime who did it in the full knowledge that they were using illegal weapons in a barbaric attack on their own people."

MrJohnson added: "You cannot go on with a regime that's willing to use illegal weapons against its own people, a regime that's killed hundreds of thousands of its own people.

"What's needed now is a political process to get rid of that regime and give the people of Syria a chance."

Boris JohnsonCredit:
Richard Drew

The US, Britain and the EU have all blamed the Syrian government for the attack in Idlib province, which happened hours before the start of a donor conference on Syria in Brussels.

President Donald Trump described the slaughter as a “heinous” act that “cannot be ignored by the civilised world”.

On Tuesday Mrs May said she was "appalled" by the attack, adding: "If proven, this will be further evidence of the barbarity of the Syrian regime".

A Syrian man in taken by civil defence workers following a suspected toxic gas attack Credit:
MOHAMED AL-BAKOUR /AFP

She went on: "I'm very clear that there can be no future for Assad in a stable Syria which is representative of all the Syrian people and I call on all the third parties involved to ensure that we have a transition away from Assad. We cannot allow this suffering to continue."

The Syrian armed forces have “categorically denied” responsibility, while Russia claimed a Syrian air strike had hit a “terrorist warehouse” containing a stockpile of “toxic substances” intended for use in Iraq.

Russia's defence ministry claimed on Wednesday that the poisonous gas contamination was the result of gas leaking from a rebel chemical weapons depot after it was hit by Syrian government air strikes.

"Yesterday (Tuesday), from 11:30 am to 12:30 p.m. local time, Syrian aviation made a strike on a large terrorist ammunition depot and a concentration of military hardware in the eastern outskirts of Khan Sheikhoun town," Russian defence ministry spokesman Igor Konoshenkov said in a statement posted on YouTube.

"On the territory of the depot there were workshops which produced chemical warfare munitions."

Labour MP John Woodcock condemned the lack of resolve from the Government, saying the use of chemical weapons would be "legitimised" if the West failed to act.

Mr Woodcock said: "Unless you take a meaningful stand against the use of chemical weapons then you effectively legitimise their use and that is a terrible thing for the security of the world and for the future victims of these hideous weapons.

"These weapons have been outlawed for 100 years for good reason. For all the difficulties of there being a military retaliation, if all we do is pass a strongly-worded resolution, or say something vague about future consequences, then we are effectively saying anything goes. The consequences of that are more frightening than the difficulties of doing something now."